Front Fluid Quantity and Suspension Response

My latest K75RT, a 92, had Showa forks. This is the first I had Showa's. The front of the bike was scary under braking. It dove like crazy. I thought I needed new Progressive Springs.

As part of the prep for running it to the West, I drained and refilled the forks. For some reason, I checked the amount of fluid that came out of the first fork, ~360 cc's. Odd number it seemed. The Non-Showa RT's took 280 cc, and the non-sport suspension was supposed to take 330 cc and the drained amount was more than this.

I checked the owners manual. It said 280 and 330 cc's. Hmmm. I checked the manual version date. It was old... and pre Showa... Time to call the dealer.

Then I rode the bike. Even I, the suspension idiot, could tell the difference. :):) It was a night and day difference. The dive was much reduced!! The front end suspension felt much more composed.

Then I thought about it a bit. The new, proper, larger fluid volume, reduces the air volume above it, compared to the way I got the bike. This means that when the fork compresses, the air PRESSURE increases much faster than when the volume is larger. Increasing air pressure acts like a rising rate spring. Effectively, putting more fluid in the fork increased the spring rate a lot when the fork tried to dive. (Some time back, HD had a brake dive reduction scheme, by isolating an air chamber that connected to above the fork oil. It gave a "higher" spring rate, when you were on the brakes and the fork air volume was cut off from the chamber.)

Having the proper amount of fluid in the forks IMO, eliminated the need for a spring transplant and made the CC jaunt much more pleasurable.

So, if you want more "cushiness" and "plushness" although with more dive, put in less fluid. If you want more "control" and "firmness" and "less dive" put in more fluid. Remember there is a limit on how low or high you can go. If you go too low, you can hear the transition from air to fluid as the fork valving compresses. Shock "absorption" will vary a lot depending if there is air or oil going through the restriction orifices. If you go too high, the fork tube pressure will go REAL HIGH. The suspension may appear to be "solid" or "locked". You might be able to blow the fork seals or O-rings out. Either are not be good things. Remember, when you deviate from the BMW factory values, you are on your own. Travel with care.

BWM specified fork oil quantities/capacities have varied over the years. Here is a table of values: